1. Field of the Invention
The present invention relates to a passive optical network system and an optical line terminator, and particularly to a passive optical network system in which plural subscriber connecting equipments share an optical transmission line and an optical line terminator.
2. Description of the Related Art
In order to transmit and receive a large volume of image signals and data through a communication network, speed-up and broadening of the communication network is performed also in an access network for connecting a subscriber to the communication network, and the introduction of a passive optical network system (hereinafter referred to as PON) specified by Recommendation G.984.1-3 or the like of International Telecommunication Union (hereinafter referred to as ITU-T) is performed. The PON is a system in which an optical line terminator (hereinafter referred to as OLT) connected to an upper communication network and optical network units (hereinafter referred to as ONUs) containing terminals (PCs or phones) of plural subscribers are connected to each other through a passive optical network including a trunk optical fiber, an optical splitter and plural branch optical fibers. Specifically, communication is performed in such a form that signals from terminals (PCs etc.) connected to the respective ONUs are sent, as optical signals, from the branch optical fibers to the trunk optical fiber through the optical splitter, are optically (time division) multiplexed on the trunk optical fiber, and are sent to the OLT, and the OLT performs communication processing on the signals from the respective ONUs and sends them to the upper communication network, or sends them to another ONU connected to the OLT.
The development and introduction of the PON starts from a system to handle a low speed signal of, for example, 64 kbit/sec, and the introduction of Broadband PON (BPON) to transmit and receive a fixed length ATM cell at a maximum speed of about 600 Mbit/sec, Ethernet (registered trademark) PON (EPON) to transmit and receive a variable length packet of Ethernet at a maximum speed of about 1 Gbit/sec, or Gigabit PON (GPON) which handles a higher speed signal of about 2.4 Gbit/sec and is standardized by ITU-T Recommendation G.984.1, G.984.2 and 984.3 is advanced. Further, in future, a high speed PON which can handle a signal of 10 Gbit/sec to 40 Gbit/sec is requested to be realized. As means for realizing such high speed PON, multiplexing methods such as Time Division Multiplexing (TDM) to time-division multiplex many signals, Wavelength Division Multiplexing (WDM) to wavelength-multiplex them, and Code Division Multiplexing (CDM) to code-multiplex them are studied. Incidentally, the present PON adopts the TDM, and for example, the GPON has such a structure that different wavelengths are used between the upstream (from the ONU to the OLT) signal and the downstream (from the OLT to the ONU) signal, and with respect to the communication between the OLT and the respective ONUs, signal communication time is assigned to each ONU. Besides, from the structure of the related art in which fixed length signals are processed, the structure becomes such that burst-like variable length signals (burst signals) in which more various signals (audio, image, data, etc.) are easily handled are also processed. With respect to a future high speed PON, although various multiplexing methods are studied as described above, the study in the direction of adopting the TDM becomes main.
In the mode of each of the PONs, since the ONUs are installed in subscriber homes existing at various places, the distances from the OLT to the respective ONUs are different from each other. That is, since the length (transmission distance) of the optical fiber including the trunk optical fiber and the branch optical fiber from the OLT to each ONU varies, a transmission delay between the OLT and each ONU varies. Thus, even if the respective ONUs transmit signals at different timings, there is a possibility that the optical signals outputted from the respective ONUs collide and interfere with each other on the trunk optical fiber. Thus, in each of the PONs, the distance between the OLT and the ONU is measured by using, for example, the technique called ranging as specified in Chapter 10 of G.984.3, and the delays of the output signals of the respective ONUs are adjusted so that the signal outputs from the respective ONUs do not collide with each other.
Further, when the OLT determines the bandwidths of signals, which are allowed to be transmitted by the ONUs, based on transmission requests from the respective ONUs by using the technique called dynamic bandwidth assignment (hereinafter referred to as DBA), the delay amounts measured by the ranging are also considered, and the transmission timings are specified for the respective ONUs so that the optical signals from the respective ONUs do not collide and interfere with each other on the trunk optical fiber. That is, the PON is structured such that the operation of communication is performed in the state where the timings of signals transmitted and received between the OLT and the respective ONUs are managed in the system.
In the transmission and reception of signals between the OLT and the respective ONUs, for example, according to the regulation in Chapter 8.3.3 of G.984.2, at the head of the signal from each ONU, a guard time for interference prevention including a maximum of 12 bytes, a preamble used for determination of a signal identification threshold of a receiver in the OLT and for clock extraction, a burst overhead byte called a delimiter to identify a separator between received signals, and a control signal (also called an overhead or a header) of the PON are added to data (also called payload), so that the OLT can identify and process the signals sent from the respective ONUs and multiplexed on the trunk optical fiber. Incidentally, since the respective data are variable length burst data, a header called a G-PON Encapsulation Method (GEM) header for processing the variable length data is also added to the head of each data. With respect to a signal from the OLT to each ONU, at the head of the signal transmitted from the OLT to each ONU, a frame synchronization pattern to identify the head, a PLOAM field to transmit monitor, maintenance and control information, and an overhead (also called a header) called a grant designation field to instruct signal transmission timing of each ONU are added to data time-division multiplexed for each ONU, so that each ONU can identify and process the signal from the OLT. Incidentally, similarly to the signal from the ONU, a GEM header for processing variable length data is added to the data multiplexed for each ONU. The OLT uses the grant designation field and assigns an upstream transmission allowance timing (transmission start (Start) and stop (Stop)) of each ONU to each ONU in units of bytes. The transmission allowance timing is called the grant. When the respective ONUs transmit data to the OLT at the allowance timings, these are optically (time division) multiplexed on the optical fiber and are received by the OLT.
Non-patent document 1: ITU-T Recommendation G.984.1
Non-patent document 2: ITU-T Recommendation G.984.2
Non-patent document 3: ITU-T Recommendation G.984.3